The last influential paper dealing with single measurements of metal abundances is [1]}, presenting an analysis of the ASCA spectra of 273 groups and clusters with the largest possible aperture collecting all the detectable flux and stacked in bins of temperature. That work found a constant Fe abundance value of 0.3 Solar for hot clusters and for groups with an increase up to a factor of 3 with respect to the average value in the range 2-4 keV.
This is a manifestation of the "inverse" Fe-bias [2]}, [3]}, [4]} which overestimates the abundances in multi-temperature plasma (ranging from about 1-2 keV to about 5 keV) resulting in a mean global temperature in the range 2-4 keV as found by [1]}. Although in practice when this occurs the spectra show the presence of both Fe-L and Fe-K lines, the inverse Fe-bias is essentially weighted by the higher statistics of the Fe-L complex. In this regime the fitting procedure increases the estimated Fe abundance to overcome the weaker Fe-lines expected in the single temperature plasma [4]}.
